"plastic VW crest/button, contact disk, and spring."
The contact disk is performing the electrical chores, hence the plastic VW button/crest.
Do just as Paul says. The crest isn't really being mounted as such, it's being placed/replaced into a moving assembly of bits and pieces. It's held in it's place by that assembly. Fancy words for two chunks of plastic a spring and a copper disk . . .
There is one other issue, depending upon who manufactured the Porsche hub cap crest, SOME of them as a little smaller than the others. The smaller ones are a perfect fit, the larger ones will need a tiny notch cut into the edge, just like you'll find on the VW crest.
The VW plastic base/housing as a vertical cast into in, and the plastic crest/button has a corresponding notch in IT/ This is done to keep the crest aligned upright so that the Wolfsburg Castle doesn't topple over or lean like the Pisa Tower. It also prevents lateral movement when depressing the horn button.
It's a quick zip with a Dremel wheel, you'll know if you need to do it when assembling the whole thing.
A last note: If the assembly is "tight" after pre-assembly. If there isn't enough up and down movement when you depress the button, or if it's binding a bit, you can remove the contact disk entirely. Since the Porsche crest is metal rather than plastic as the VW one is, AND it's flat rather than domed as the VW one is, it'll work fine without the contact disk. I didn't put the disk back in either of the ones that I've already made and they work PISSA!
Your choice, either way, no biggie.
Seems like a lot of words for a tiny task, once you open up the assembly you'll see how simple it all is. If you need a photo of an open assembly just post and I'll shoot one for you.